The Rare Defense Gem, previously known as the Rainbow Defense Gem, is a diamond gem created by fusing all eight elemental defense gems together in the proper order (1. forge: fire, ice, water; 2. forge: shock, poison, wind; 3. forge: gems from 1. in 2. forge and one hexagon gem). It grants defense to all elements in the game (prism defense) and sells for 20,000. Forging three rare defense gems together produces a Rare XP Gem which grants 1,000 xp on every hit. It is the only rare gem that only needs low costing equipment to make it.

Note: Make sure you have no store-bought dark or light defense gems in your inventory before starting the process, or you may get them mixed up with the ones you craft. Only gems forged manually will yield the rare defense gem.

All of the required gems, aside from dark and light (which both need to be crafted), can be purchased in the store. You can also spin gem wheels if you cannot obtain a particular defense gem.

Fuse fire, ice, and water defense gems together (in that order) to get a light defense gem. All of them can be bought from the store for 5,000. The forge process takes 1 month game time or 30 minutes real time.

Fuse shock, poison, and wind defense gems together (in that order) to get a dark defense gem. All of them can be bought from the store for 5,000. The forge process takes 1 month game time or 30 minutes real time.

Fuse the light and dark defense gems (must be the forged ones) together with anyhexagon gem (in that order) to get the rainbow defense gem. The forge process time changes depending on the level of the hexagon gem, with a cheaper gem resulting in a quicker forge process, and a more expensive one increasing the amount of time it will take for it to be completed. It is recommended you buy a low-cost hexagon gem from the store for this forge.

Upon obtaining the rare defense gem, the "Solar Fusion" achievement is unlocked.

If you forge three Rare Defense Gems together, the result will be a Rare XP Gem.

In Infinity Blade III, Rare Defense Gems vary from 5% to 100% elemental resistance. Higher level gems are created by fusing together lower level gems. The lowest level (5% resistance) rare defense gems are created by fusing together three regular resistance gems. Those can be store-bought and of any type.

Overall, it takes 243 regular resistance gems to produce one 100% rare defense gem. The store always sells 2 or 3 resistance gems on each awakening, each costing 5000 gold. It's prudent to start buying and fusing them as early as financially possible, because the whole process is quite lengthy.

It is a good idea to make two of these, one for Siris and one for Isa. Also, if you lose one in Deathless Mode, you can still equip the other instead of making a new one.

Elemental resistance gems are cumulative: their effects add up linearly all the way up to 100%. For example, complete resistance can be achieved by using two 50% defense gems: 50% + 50% = 100%. It should be noted that 100% resistance allows the player to block attacks from non-Spectrum enemies without taking damage, making shields viable even without mastering perfect blocking. Strikes from enemies with Elemental Titan perk or otherwise can use Spectrum attacks will count as "broken block", but will inflict very little damage with a minimal duration (this is similar to how Spectrum resistant titans still take Spectrum damage, but only at 10% vulnerability). Pushing resistance over 100% doesn't prevent this damage.